Modem base metal smelters capture the bulk of sulfur dioxide gas (SO.sub.2) and flue dusts from off-gases produced by smelting. Off-gas cleaning by aqueous scrubbing typically produces acidic solutions rich in SO.sub.2 and dissolved metals such as chromium, cobalt, copper, iron, lead, nickel, zinc, etc., and dissolved metalloids such as selenium, tellurium, arsenic, etc. The concentrations of some of the dissolved species in the scrubber solution may be above governmental water management guidelines (see, e.g., "Water Management Policies, Guidelines, Provincial Water Quality Objectives", Ministry of Environment and Energy, Ontario, Canada, July 1994). Removal of these species from scrubber solution is therefore desirable. Removal of many of these species may be accomplished by precipitation via neutralization using, e.g., lime (see, e.g., Jackson, E., Hydrometallurgical Extraction and Reclamation, Wiley, Toronto, 1986). However, neutralization alone is typically not effective for the removal of dissolved selenium, tellurium and arsenic. Smelter scrubber solutions, for example, can contain 1 to 60 milligrams per liter (mg/L) of dissolved selenium or more in the form of selenite (Se(IV)) ions and/or selenate (Se(VI)) ions, 1 to 5 mg/L or more of dissolved tellurium in the form of tellurite (Te(IV)) ions and/or tellurate (Te(VI)) ions, and 50 to 230 mg/L or more of dissolved arsenic in the form of arsenite (As(III)) ions and/or arsenate (As(V)) ions.
Procedures for the removal of dissolved arsenic from scrubber solutions are disclosed in U.S. Pat. No. 5,820,966 to Krause et al. This process involves oxidizing the components of the solution by addition of, e.g., air, followed by neutralization by addition of, e.g., slaked lime in the presence of a sufficient quantity of dissolved ferric (Fe(III)) ion. The Fe(III) ion co-precipitates with dissolved arsenic to produce an environmentally preferred solid iron-arsenic compound, thus reducing the concentration of dissolved arsenic in the solution. A reduction in the concentration of many dissolved metals in solution also occurs. However, this process by itself is not effective for removal of selenium from solutions containing significant concentrations of dissolved selenium, and Se(VI) ions in particular.
A known process for removing selenium ions from solution involves providing SO.sub.2 and heat to a solution containing Se(IV) ions to reduce the Se(IV) ions to elemental selenium (see, e.g., Kudryavtsev, A. A., The Chemistry and Technology of Selenium and Tellurium, Collet's, London, 1974). The process requires long residence times and is not effective for the removal of Se(VI) ions from solution.
U.S. Pat. No. 4,405,464 to Baldwin et al. discloses a process for the removal of Se(VI) ions from aqueous solution by contact with metallic iron at a solution pH adjusted to below about 6.0. The examples in the Baldwin et al. patent pertain to the treatment of solutions containing less than about 0.5 mg/L of Se(VI). This process is not effective for reducing high concentrations of Se(VI).
U.S. Pat. No. 4,806,264 to Murphy discloses a method of removing Se(IV) and Se(VI) ions from an aqueous solution. The method includes contacting the solution with an amount of ferrous hydroxide solids at a pH of about 8 to 10 and a preferred temperature of from about 10.degree. C. to 35.degree. C. The examples in the Murphy patent pertain to the treatment of solutions containing less than about 1 mg/L of Se(VI). However, this process is not effective for reducing high concentrations of Se(VI).
The effectiveness of the above prior art processes for the removal of dissolved tellurium from solution is not known.
There is a need for new, efficient methods that are capable of removing dissolved metals and metalloids from aqueous solutions, particularly those methods capable of removing relatively high concentrations (e.g., greater than 1 mg/L) of dissolved metalloids such as selenium, tellurium and arsenic.